High Speed Wire Cutting Needs High Efficiency Filtration

High speed cutting with wire electrical discharge machining produces debris in the dielectric at a high rate. Without effective filtration, a wire machine quickly loses its ability to maintain high cutting speed, achieve the desired surface finish and obtain expected dimensional accuracy. Charmilles' approach to solving this problem has been to develop a filtration system that doubles the number of filter cartridges, from two to four.

According to its developers, the Robofil 240cc and 440cc wire EDM models are capable of cutting tool steel at 42 square inches per hour under realistic shop conditions. Currently, these are the fastest cutting wire machines from Charmilles Technologies Corp. (Lincolnshire, Illinois). Other builders are also offering wire machines that cut at astonishing speeds, making it a highly competitive market.

One of the challenges created by high speed wire EDM is developing filtration systems that can keep up with the high rate at which debris enters the dielectric fluid. The quantity of this debris is not the only issue. Removing it from the dielectric water quickly, effectively and thoroughly is also critical. Without effective filtration, a wire machine quickly loses its ability to maintain high cutting speed, achieve the desired surface finish and obtain expected dimensional accuracy.

Charmilles’ approach to solving this problem has been to develop a filtration system that doubles the number of filter cartridges, from two to four. The system arranges the filter cartridges in two circuits that run independently or simultaneously. All four filters are used during peak metal removal (very tall cuts, for example). At other times, one circuit can be cut out for replacing spent paper filtering elements without interrupting the EDM process. Two of the cartridges are located directly in the worktank. These filters draw in water in such a way that constant pressure is maintained on the filter element, forcing particles to be trapped more evenly and densely across the filter surfaces. All of the cartridges use paper filtering elements with a new design that increases surface area about 54 percent, boosting the capacity to trap particles.

Charmilles reports that doubling the number of filters more than doubles the life of each filter. Filter elements tend to last 2.6 times longer under similar operating conditions, allowing the machine to run unattended 3.8 times longer. This reduces filter costs by 40 percent despite having twice the number of filters in the filtration system.

Effective filtration means that only water that is virtually free of particles flows into the spark gap, the minute space between the electrode wire and the workpiece surface. The absence of particles allows sparks to more consistently follow the precise pattern of rising and falling energy levels set by the generator. The Robofil 240cc and 440cc use Charmilles’ new CC-Generator, which is capable of producing an increased ignition voltage and higher maximum amplitude. In part, this spark “shape” enables these machines to remove material very efficiently.

Dielectric fluid with a minimum number of particles cools the wire more effectively, too, reducing heat stresses that can cause wire breakage. This is especially important on tall cuts and steep tapers (the 440cc can cut a workpiece as tall as 24 inches and taper at 30 degrees maximum at full height).

Finally, the spark gap is not the only place where unwanted particles have a negative effect. When particles reach the power contacts that allow the wire to be energized, sparks can occur, causing erosion that shortens the life of the contacts. Excessive particles can cause premature wear of the seals that allow the lower guide arm to move without leakage of the worktank. This makes the fixed table design of the machines practical, which in turn enables them to load very heavy parts without affecting accuracy, the company says.